1. Field of the Invention
The present invention relates to an optical module used for optical fiber communications.
2. Related Art
As optical communication systems have been developed to have larger capacity and operate at higher speed, wavelength division multiplexing communications (D-WDM) are adopted in trunk-line optical communication systems. With such developments, a highly reliable optical module which enables a wavelength control with high accuracy is required as an optical module for D-WDM.
Japanese Patent Laid-Open Publication No. 8-94875 discloses such an optical module, in which an optical unit is accommodated in a package made of a material having a low linear expansion coefficient. According to this optical module, as the thermal expansion and the thermal contraction caused by temperature changes in the package are small so that a stress applied to the optical unit is reduced, it is possible to prevent breakage or an increase in loss variations of optical elements integrated on the optical unit.
On the other hand, in many optical modules for D-WDM, optical elements are operated in a constant temperature using a temperature control element in order to suppress variations in the optical wavelength and the like caused by the changes in the characteristics of the optical elements due to the environmental temperature variation.
Further, as a planar lightwave circuit (PLC) is in the form of a thin chip and the light emitting position thereof is too low to provide light coupling with a lens or with an optical fiber, the height of the optical axis is required to be adjusted. As such, a typical optical module is configured such that an optical unit in which optical elements are mounted on a carrier and the optical axis position of emitted light is adjusted is accommodated in a package. The carrier has a function of improving the mechanical strength of the optical unit and also lowering the heat resistance between the temperature control element and the optical elements to thereby perform constant-temperature operation. Therefore, a firm and highly-thermal conductive material is demanded for the carrier.
For joining a PLC and a carrier, soldering is used for not interrupting conductivity of the joined surfaces. FIGS. 3A and 3B show a conventional optical module in which an optical element and a carrier are soldered. In the optical module of FIGS. 3A and 3B, a substrate 1 in which a ring optical resonator 2 including a PLC and an SOA element 3 are mounted on a carrier 4, and the substrate 1 and the carrier 4 are soldered via a solder joint surface 2A.
However, in soldering the substrate 1 on the carrier 4, it is required to raise the temperature to the melting point of the solder. As the temperature rises, the substrate 1 and the carrier 4 expand according to the properties of the respective materials, and are soldered in the expanded state. When the temperature decreases from such a state to a room temperature for fixing the solder joint, the substrate 1 and the carrier 4 contract according to the properties of the respective materials. If the contraction rates of the substrate 1 and the carrier 4 which are fixed by soldering differ from each other, the substrate 1 will warps with the carrier 4 due to a bimetallic effect as shown in FIG. 3B.
The carrier may be made of copper tungsten (CuW), kovar, or aluminum nitride (AlN) for example, and the linear expansion coefficient of Cu(20)-W is 8.5*10−6/K, that of kovar is 5.3*10−6/K, and that of AlN is 4.5*10−6/K. Generally, a substrate configuring a PLC is made of Si having a linear expansion coefficient of 4.2*10−6/K which is significantly different from that of CuW or kovar. Although Si and AlN have a relatively small difference in their linear expansion coefficients, if the substrate and the carrier are soldered, a bimetallic effect will be caused as shown in FIG. 3B, so that the stress affects the PLC.
In such an optical module, wavelength variations are caused by the warpage in the substrate configuring the PLC, which causes a problem that the characteristics of the PLC will be changed after the substrate configuring the PLC is mounted on the carrier.
Further, as the warpage in the substrate and in the carrier remains therein as a thermal contraction stress, it is known that the residual contraction stress is released in a high temperature environment or in a thermal cycle environment so that the warpage quantity decreases. As such, there is also a problem that the characteristics of the PLC change during the substrate being mounted on the carrier, in addition to the fact that the characteristics thereof change after it is mounted on the carrier. As such, it is required to reduce the initial thermal contraction stress as much as possible.